Apparatuses to electrically power the electrodes of an electric arc furnace for melting metal are known.
The electrodes are put in the electric furnace and can be moved inside it, toward/away from the metal bath, to regulate the entity of melting the solid metals.
Known electric power apparatuses comprise devices to regulate the position of the electrodes which allow to lift or lower the electrodes with respect to the metal in order to modify the length of the arc. However, regulating the position of the electrodes is rather slow, and does not allow to carry out rapid corrections if there are sudden changes in the electric power quantities or in the generation of the electric arc.
In fact, if the arc gets longer, that is, if the electrode is farther from the mass to be melted, the voltage applied has to be increased in order to support it and prevent it from switching off; if the arc is shortened, the current increases and therefore there is an uncontrolled increase in the power supplied by the arc, with possible damage to the furnace or the roof.
The arc voltage, given the same current, is proportional to the distance between the electrode and the mass to be melted. Therefore, for a given functioning condition, which provides to reach a reference current of the arc, a stable arc condition is obtained by regulating the distance between the electrode and the mass to be melted, in relation to a characteristic curve of power voltage.
Power apparatuses of the known type generally comprise a multi-tap transformer that transforms the voltage supplied by an electric power network, generally average voltage, into a voltage suitable to power the electrodes.
Through the taps of the transformer it is possible to regulate the voltage of the arc, and hence its length, to regulate the melting power.
These types of power apparatuses have a disadvantage connected to the discontinuity of instantaneous absorption of energy, which is taken from the power network and which occurs particularly at the start of melting due to the frequent short circuits caused by the instability of the arc and the slippage of the scrap. In some cases, this can entail fluctuations in the network voltage (this phenomenon is also called flicker).
Power apparatuses for electric arc furnaces are also known which comprise a straightener circuit, which transforms the alternate current supplied by the power network into direct current, and a circuit of convertors to power the electrodes.
One solution that uses a general configuration of this type to power an electric arc furnace is described for example in the patent application US2007/0247079A1 and in the patent U.S. Pat. No. 6,421,366 B1.
However, the convertor circuits present in these power apparatuses do not allow to compensate the disturbances arriving from the arc and which affect the power network. Indeed, the convertor circuits, because of the modulation of the current that is performed, generate harmonics in the current that can be harmful if introduced into the power network.
One purpose of the present invention is to obtain an apparatus and method to power an electric arc furnace that allows to regulate the melting power thereof efficiently.
Another purpose of the present invention is to obtain an apparatus and implement a method that allow to regulate the characteristics of the arc voltage and the arc current to guarantee the stability of the electric arc during melting.
Another purpose of the present invention is to obtain a power apparatus that reduces the disturbances induced in the power network compared with apparatuses of a known type.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.